[SOLVED] use 2n2222a as switch

[dxtr]

hey everybody
i need to use 2n2222a as a switch to drive 5V , the transistor will be controlled by a Microcontroller who deliver 2.7 V
my question is : is 2.7 enough to drive the Transistor ? because i didn't work for me in reality but works fine in Isis

 

[crutschow]

2.7V is sufficient if the microcontroller can supply adequate output current (the transistor ON base voltage is typically ≈0.7V). The transistor base current should be at least 10% of the collector current.

What is the 5V transistor load?

 

[Centmo]

Depends on the max output current from your MCU I/O pin, and as mentioned above, the maximum current through the transistor. I'd use a MOSFET personally.

 

[dxtr]

2.7V is sufficient if the microcontroller can supply adequate output current (the transistor ON base voltage is typically ≈0.7V). The transistor base current should be at least 10% of the collector current.
What is the 5V transistor load?

The output current of my µC is 1mA , do you think its enough

Depends on the max output current from your MCU I/O pin, and as mentioned above, the maximum current through the transistor. I'd use a MOSFET personally.

Why Mosfet is better , and can you recommend me a good one ?

 

[Centmo]

MOSFETs don't require a constant current on the gate to keep them on, you just charge the gate capacitance up and it turns on and stays on. Therefore, it would be OK to drive a small one from an MCU. A recommendation depends on your maximum voltage that you are switching, and what your load is (to determine a maximum drain current). You can go to digikey and find one based on these search criteria.

 

[dxtr]

MOSFETs don't require a constant current on the gate to keep them on, you just charge the gate capacitance up and it turns on and stays on. Therefore, it would be OK to drive a small one from an MCU. A recommendation depends on your maximum voltage that you are switching, and what your load is (to determine a maximum drain current). You can go to digikey and find one based on these search criteria.

Thnx , now i just need to find a MOSFET that can turns on with 2.7V and 1mA

 

[Centmo]

Any MOSFET will turn on with 1mA...eventually. It is just a matter of how fast it will turn on. What are you trying to power through the FET?

 

[crutschow]

Most MOSFETs require more than 2.7V to fully turn ON, so the 2N2222A is likely better as long as the transistor collector load current is no more than about 10 to 20 mA.

 

[Centmo]

True for most FETs, crutschow, but I just had a peek on digikey, and there are plenty that have quite a low Vgs threshold. This is the first one that popped up in a long list:

Si2302 (NFET)
https://www.vishay.com/docs/70628/70628.pdf
Vds rated = 20V
Id max = 2.4A (at Vgs = 2.5V)
Vgs(threshold) = 0.65V
Gate leakage: 100nA

https://www.digikey.ca/product-detail/en/SI2302CDS-T1-E3/SI2302CDS-T1-E3TR-ND/2345137
cost 9 cents each. $8 for shipping any sized order (for Canada anyway).

Again, it depends on what you're trying to switch on/off with this device. 10-20mA is enough for a single LED but nothing more.

 

[dxtr]

The load is a 4051 8 chanel analog multiplexer , the high level of the selector pins A,B and C is 5V , i can't use my µC (2.7 output) that's why i need to use a Transistor as a switch

 

[Centmo]

Oh, in that case it will draw pretty much no current. You could do the same thing with a smaller, cheaper device, but the 2n2222a will be fine. I don't know if there's a MUX that can run from 2.7V signals but it would be worth a look to reduce parts count.

Edit: Actually, I am looking at the datasheet for the 74HC4051, and you can power this IC from 2.7V and use just the MCU to drive the input selector pins. Otherwise, you can use the 74HCT4051 and power it from 5V and still drive with your 2.7V outputs. A high level is over 2.0V max (1.6V typ), and a low level is below 0.8V min (1.2V typ). So, you wouldn't need your 2n2222a afterall.

**broken link removed**

 

[dxtr]

Oh, in that case it will draw pretty much no current. You could do the same thing with a smaller, cheaper device, but the 2n2222a will be fine. I don't know if there's a MUX that can run from 2.7V signals but it would be worth a look to reduce parts count.Edit: Actually, I am looking at the datasheet for the 74HC4051, and you can power this IC from 2.7V and use just the MCU to drive the input selector pins. Otherwise, you can use the 74HCT4051 and power it from 5V and still drive with your 2.7V outputs. A high level is over 2.0V max (1.6V typ), and a low level is below 0.8V min (1.2V typ). So, you wouldn't need your 2n2222a afterall.**broken link removed**

Thank you so much for your help , well its works very fine right now using the mosfet , i didn't find the 74HCT4051 he only gives me 4051 (that what is writing in the shield) and i couldn't drive it with 2.7 directly , now it is driven with 5V using mosfet and works just fineThank you so much for your support